In recent years, various bone diseases, such as osteoporosis, bone fractures, lumbago or the like have increased along with the progressive increase in the elderly population. In a bone tissue, osteogenesis and bone resorption incessantly occur. In a young person, a balance between osteogenesis and bone resorption is kept, but the balance is disrupted to bone resorption owing to various causes with aging (uncoupling). Continuance of this state for a long period of time makes the bone tissue fragile, resulting in occurrence of various bone diseases, such as osteoporosis, bone fractures, and lumbago. It is considered that prevention of the uncoupling enables prevention of various bone diseases, such as osteoporosis, bone fractures, and lumbago.
Conventionally, in order to prevent the uncoupling to prevent or treat bone diseases, the following methods have been performed: (1) calcium supplementation by diet, (2) light exercise, (3) insolation, (4) medication, and the like. For calcium supplementation by diet, there are used calcium salts, such as calcium carbonate, calcium phosphate or the like, or natural calcium agents, such as eggshell, fish bone powder or the like. However, these materials are not necessarily suitable for oral intake. Jogging, walking, or the like may be recommended as light exercise. However, even light exercise is troublesome for a person whose body has weakened, and it is almost impossible for a bedridden old person to do exercise. It is considered that insolation is a good means to supplement activated vitamin D3, but it is not sufficient in itself. 1α-Hydroxyvitamin D3, a calcitonin preparation, or the like is used for administration of a pharmaceutical, and is known to be effective for treating osteoporosis. However, these substances are pharmaceuticals themselves and cannot be used as a food material.
The inventors of the present invention have searched for a bone-strengthening factor contained in milk in order to obtain a bone-strengthening substance that can be used as a food material. As a result, the inventors found that a protein and a peptide mixture obtained by removing a salt derived from a milk serum from a water-soluble fraction of a milk serum protein exhibit a bone-strengthening effect (see Patent Document 1, for example). The inventors found that a fraction obtained by subjecting an aqueous solution of the protein and the peptide mixture to an ethanol treatment, a heat treatment, a salting treatment, and an ultrafiltration membrane treatment exhibits an osteoblast growth promoting effect and a bone-strengthening effect (see Patent Documents 2 and 3, for example). The inventors further found that a basic protein contained in milk exhibits an osteoblast growth promoting effect, a bone-strengthening effect, and a bone resorption prevention effect (see Patent Document 4, for example).    Patent Document 1: Japanese Patent No. 3160862    Patent Document 2: Japanese Patent No. 3092874    Patent Document 3: JP-A-H05-320066    Patent Document 4: Japanese Patent No. 3112637